[CH3:1][O:2][C:3](=[O:17])[C:4]1[CH:9]=[C:8]([OH:10])[C:7]([Br:11])=[C:6]([OH:12])[C:5]=1[CH2:13][C:14]([CH3:16])=[CH2:15].B(F)(F)F.CCOCC.O>C(Cl)Cl>[CH3:1][O:2][C:3]([C:4]1[CH:9]=[C:8]([OH:10])[C:7]([Br:11])=[C:6]2[O:12][C:14]([CH3:16])([CH3:15])[CH2:13][C:5]=12)=[O:17]To a stirred solution of 4-bromo-3,5-dihydroxy-2-(2-methyl-allyl)-benzoic acid methyl ester (197c) (85 g, 0.28 mol) in CH2Cl2 (1500 mL) was added drop wise BF3.Et2O (180 mL, 1.41 mol) at 0° C. The mixture was stirred at room temperature for 2 hr. After TLC (petroleum ether/EtOAc=5/1) showed the reaction was complete, the reaction mixture was poured into water (1 L). The organic phase was separated and washed with saturated aqueous NaHCO3, dried over Na2SO4 and concentrated to give a yellow solid. The crude solid was purified by column chromatography (petroleum ether/EtOAc=15/1) to give the title compound (37 g, 44% yield) as a white solid. 1H NMR (400 MHz, CDCl3) δ 7.07 (s, 1H), 5.43 (s, 1H), 3.80 (s, 3H), 3.29 (s, 2H), 1.45 (s, 6H)
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